ORGANIC LETTERS
Selectivity Enhancement of Silica-Supported Sulfonic Acid Catalysts in Water by Coating of Ionic Liquid
2007 Vol. 9, No. 16 3145-3148
Yanlong Gu, Ayman Karam, Franc¸ ois Je´roˆme,* and Joe1 l Barrault Laboratoire de Catalyse en Chimie Organique, UMR 6503, CNRS-UniVersite´ de Poitiers, ESIP, 40 AVenue du Recteur, Poitiers Cedex 86022, France
[email protected] Received June 8, 2007
ABSTRACT
Coating of silica-supported sulfonic acid catalysts with hydrophobic ionic liquid leads to a significant improvement of catalyst selectivity. Many organic reactions, including Prins cyclization, cycloaddition of epoxide to aldehyde, and dehydrative etherification of secondary benzyl alcohols, proceed well in formalin or pure water. In particular, tandem dehydration/Prins cyclization reactions of tertiary and secondary alcohols with formaldehyde were developed for the first time.
Immobilization of molecular catalysts onto a solid support is a promising strategy to facilitate the separation of the catalyst from the products. However, this approach generally decreases the catalytic performance, including the activity and/or selectivity. To solve this problem, researchers deliberately modified the surface of catalysts to be hydrophobic, hydrophilic, or amphiphilic with the hope of accelerating approach of substrate to the active centers.1 Even if such approaches were efficient to improve the catalytic activity, these strategies were far less efficient to improve the selectivity of solid catalysts, which still remains a huge challenge for heterogeneous catalysis. For the moment, one (1) See some recent examples for hydrophobic derivatization: (a) Liu, P. N.; Gu, P. M.; Wang, F.; Tu, Y. Q. Org. Lett. 2004, 6, 169. (b) Zhang, H.; Zhang, Y.; Li, C. J. Catal. 2006, 238, 369. (c) Yang, Q.; Ma, S.; Li, J.; Xiao, F.; Xiong, H. Chem. Commun. 2006, 2495. (d) Ahu, B. Y.; Seok, S. I.; Baek, I. C.; Hong, S.-I. Chem. Commun. 2006, 189. For hydrophilic derivatization: (e) Bass, J. D.; Anderson, S. L.; Katz, A. Angew. Chem., Int. Ed. 2003, 42, 5219. (f) Notestein, J. M.; Katz, A. Chem. Eur. J. 2006, 12, 3954. (g) Bass, J. D.; Solovyov, A.; Pascall, A. J.; Katz, A. J. Am. Chem. Soc. 2006, 128, 3737. For amphiphilic derivatization: (h) Otani, W.; Kinbara, K.; Zhang, Q.; Ariga, K.; Aida, T. Chem. Eur. J. 2007, 1731. 10.1021/ol071356j CCC: $37.00 Published on Web 07/12/2007
© 2007 American Chemical Society
of the most common methods to improve the selectivity of heterogeneous catalysts is to use a shape-selective material as solid support or directly as catalyst.2 Unfortunately, most of these shape-selective systems are difficult to design and prepare. Furthermore, the substrate scope of shape-selective catalysts was rather limited because their selectivities are strictly dependent on the substrate hindrance. Recently, Kobayashi and co-workers have developed silica-supported metal catalysts with hydrophobic ionic liquids for organic reactions in water.3 It was observed that activities of silica-supported catalysts were significantly improved by coating of ionic liquids. The authors suggested that the ionic liquid creates a hydrophobic environment on the siliceous surface resulting in better diffusion of organic substrates to the catalytic sites. However, if it is now well(2) (a) Tada, M.; Iwasawa, Y. Chem. Commun. 2006, 2833. (b) Roeffaers, M. B. J.; Sels, B. F.; Uji-i, H.; De Schryver, F. C.; Jacobs, P. A.; De Vos, D. E.; Hofkens, J. Nature 2006, 439, 572. (3) (a) Gu, Y.; Ogawa, C.; Kobayashi, J.; Mori, C.; Kobayashi, S. Angew. Chem, Eng. Ed. 2006, 45, 7217. (b) Gu, Y.; Ogawa, C.; Kobayashi, S. Org. Lett. 2007, 9, 175.
established that there exists a clear synergistic effect between ionic liquids and organomodified silica, no example of selectiVity improVement resulting from this strategy was reported yet. Herein, we show that the coating of silicasupported sulfonic acids with hydrophobic ionic liquid not only increases the catalyst activity in water but also considerably improves the process selectivity in many Brønsted acidcatalyzed reactions. This association between ionic liquid and organomodified silica offers new routes for selectivity improvement of heterogeneous catalysts (Figure 1).
Figure 1. Silica materials and ionic liquid.
Preparation of silica-supported sulfonic acids (SiO2SO3H) was performed according to the reported method (see the Supporting Information).4 The obtained functionalized silica was then added to an acetonitrile solution of [C8MIm]NTf2. After 5 min of stirring at room temperature, volatile components were removed under reduced pressure affording a powdery and free-flowing solid here named SiO2-SO3HIL. The performance, activity, and selectivity of SiO2-SO3HIL were first investigated in the Prins cyclization, which is an important carbon-carbon bond-forming reaction and widely used for organic synthesis.5,6 Previously, Prins cyclization reaction of styrene has been carried out in organic solvents such as acetonitrile and 1,2-dichloroethane7 using paraformaldehyde as the HCHO source. Quite recently, Gu et al. have reported a novel hydrophobic Brønsted acidic ionic liquid (HBAIL)-catalyzed Prins cyclization of styrene derivatives using formalin as the HCHO source instead of relatively expensive paraformaldehyde.8 However, the catalytic activity of HBAIL was far from satisfactory, and the substrate scope was rather limited. Reddy et al.9 reported a Prins cyclization using well-organized mesoporous silica (SBA-15) functionalized with sulfonic acid groups as catalyst. However, this acidic mesoporous silica was unable to catalyze the reaction in formalin. We set out to examine SiO2-SO3H 1-catalyzed Prins cyclization of R-methylstyrene in formalin, and as observed (4) Li, P. H.; Wang, L. AdV. Synth. Catal. 2006, 348, 681. (5) (a) Prins, H. J. Chem. Week 1919, 16, 1072. (b) Arundale, E.; Mikeska, L. A. Chem. ReV. 1952, 51, 505. (6) Bach, T.; Lo¨bel, J. Synthesis 2002, 2521. (7) (a) Yadav, J. S.; Reddy, B. V.; Bhaishya, G. Green Chem. 2003, 5, 264. (b) Li, G.; Gu, Y.; Ding, Y.; Zhang, H.; Wang, J.; Gao, Q.; Yan, L.; Suo, J. J. Mol. Catal. Chem. 2004, 218, 147. (c) Swapna, B. S. V.; Sridhar, C.; Saileela, D.; Sunacha, A. Synth. Commun. 2005, 35, 1177. (d) Delmas, M.; Gaset, A. Tetrahedron Lett. 1981, 22, 723. (8) Gu, Y.; Ogawa, C.; Kobayashi, S. Chem. Lett. 2006, 35, 1176. (9) Reddy, S. S.; Raju, B. D.; Kumar, V. S.; Padmasri, A. H.; Narayanan, S.; Rao, K. S. R. Catal. Commun. 2007, 8, 261. 3146
by Reddy et al., a very messy mixture composed of oligomers of R-methylstyrene and their corresponding Prins adducts was produced (Table 1, entry 1). To our great delight, when
Table 1. Prins Cyclization of R-Methylstyrene in Water Using Formalin as Formaldehyde Sourcea
entry
catalyst
yield (%)
1 2b 3 4 5 6 7
SiO2-SO3H 1 SiO2-SO3H 1-[C8MIm]NTf2 no catalyst H2SO4 HCl CF3SO3H HBAILe
23c 94 0 42d 30d
